The invention relates to air compressors. More particularly, the invention relates to a method of controlling a variable speed compressor during stopping.
Conventional rotary air compressors have an inlet valve that controls air flow to the inlet or suction side of the compressor. The inlet valve throttles flow when load on the compressor is diminished and shuts fully when the load on the compressor is removed. The inlet valve is commonly referred to as an unloader valve. The compressor is loaded when the inlet valve is open permitting air to flow through the compressor inlet. The compressor is unloaded when the valve is closed to block flow through the compressor inlet.
Unloader valves are typically designed to prevent backflow through the compressor inlet. Backflow typically includes a pressurized fluid (e.g., a mixture of air and oil) and may occur when the compressor is stopped while the discharge side of the compressor is still pressurized. This negative pressure gradient allows flow out the inlet in the reverse direction.
U.S. Pat. No. 6,474,950, fully incorporated herein by reference, describes a screw compressor including a variable speed drive. Using variable frequency drive technology with air compressors allows delivery-side pressure to be controlled by varying the drive speed without the need for an inlet valve to control the system pressure. However, when an inlet valve is not utilized, backflow as described above occurs through the inlet of the compressor when the compressor is stopped.